Tools, Equipment & Processes — Moulding & Forming (AQA GCSE Design & Technology): Revision Note
Exam code: 8552
Blow moulding
Blow moulding forms a hollow thermoplastic product by inflating softened polymer inside a mould
How blow moulding works
A hot tube called a parison or an injection-moulded preform is placed in a mould
Compressed air expands it against the mould walls
The polymer cools and the mould opens
It is suited to repeated production of hollow products
Typical products and prototypes
Bottles, containers, fuel tanks and hollow toys
Waste and quality
Flash and the ends of the parison may be trimmed and recycled; lightweight wall design can reduce material use
Worked Example
Why is blow moulding suitable for making bottles like drinks bottles?
[1 mark]
Answer
It creates hollow, lightweight containers with a closed body and narrow neck in a fast, repeatable process. [1]
Casting
Casting pours or forces a liquid material into a mould where it solidifies into shape when it cools
The mould is made slightly larger than the final part to allow for shrinkage as the material cools
How casting works
Materials include molten metals, resin, plaster, concrete and some food-safe polymers
Sand casting uses a single-use sand mould; die casting uses a reusable metal mould
Risers and runners feed material and allow gases to escape
The casting is removed, cleaned and finished after solidification
Typical products and prototypes
Metal handles, resin models, jewellery and complex components
Waste and quality
Runners, risers, flash and failed castings create waste, but many metals can be remelted
Worked Example
Explain the function of a riser in sand casting.
[1 mark]
Answer
A riser provides a route for air and excess molten metal and shows when the mould cavity has filled. [1]
Examiner Tips and Tricks
When describing casting, always state that the mould is made slightly larger than the finished part, to allow for shrinkage as the molten material cools and solidifies. This shrinkage-allowance point is specifically credited in mark schemes and is easy to forget if you only describe pouring and cooling.
Injection moulding
Injection moulding forces molten thermoplastic into a closed metal mould under pressure
How injection moulding works
Granules are fed from a hopper into a heated barrel
A rotating screw melts and pushes the polymer through a nozzle
The polymer cools in the mould and ejector pins release the part
It produces complex, accurate parts at high speed after expensive tooling is made
Typical products and prototypes
Casings, toys, bottle caps, clips and electrical fittings
Waste and quality
Sprues, runners and rejected parts can often be reground; hot-runner systems reduce waste
Worked Example
Explain why injection moulding is mainly associated with mass production?
[2 marks]
Answer
The mould is expensive to design and manufacture,[1] but each moulding is produced quickly at a low unit cost once production begins. [1]
Extrusion
Extrusion forces softened material (a polymer or metal) continuously through a shaped die to create a constant cross-section
How extrusion works
Thermoplastics are heated and pushed by a screw through a die
Metals such as aluminium can be forced through a die under high pressure
The continuous length is cooled and cut to size
Only the cross-section is fixed; the length can vary
Typical products and prototypes
Pipes, window frames, cable insulation, rails and aluminium sections
Waste and quality
Start-up material and cut ends may be waste; continuous operation and accurate cutting reduce scrap
Worked Example
State one limitation of extrusion.
[1 mark]
Answer
It can only produce products with a constant cross-section along their length. [1]
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